Preheat your oven to 350°F (175°C). In a large bowl, gently mix the blackberries with sugar and lemon juice until well coated. Pour the berry mixture into your baking dish, spreading it out evenly.
In a separate bowl, combine the oats, flour, and chopped nuts (if using). Cut the cold butter into small cubes and add it to the dry ingredients.
Using a pastry cutter or forks, work the butter into the dry ingredients until the mixture resembles coarse crumbs. Some larger clumps are okay; they will create a nice crunch when baked.
Sprinkle the crumble topping evenly over the berry filling, patting it down slightly to ensure it sticks and covers the surface.
Bake in the preheated oven for about 35 minutes, or until the topping is golden brown and the filling is bubbling around the edges. Check halfway through and tent with foil if the topping starts to brown too quickly.
Remove from oven and let cool slightly before serving. The top should be crackly, and the filling soft and juicy beneath.
